There are many airborne mobile platforms that can employ one or more airfoils to supply, lift and/or thrust. In a fixed wing aircraft, for example, the wings (i.e., the airfoils) can experience relatively steady airflow. At relatively high angles of attack (i.e., orientation of the airfoil to the airflow) and/or relatively high airflow velocities, a boundary layer can sufficiently detach from a surface of the wing causing a stall condition. In the stall condition, the wings can experience a loss in lift.
Unlike wings on the fixed-wing aircraft, rotor blades of a rotorcraft can rotate with a rotor hub to which the rotor blades are connected. The rotating rotor blades are subject to cyclical variations in blade pitch angle, as well as unsteady high-subsonic airflow that can include relatively high frequency and relatively large amplitude variations in angle of attack and relatively rapid and periodic changes in an airflow velocity at one or more sections of each of the rotor blades. Rotor blades rotating through the unsteady airflow can have an increase in the maximum achievable lift (i.e., increase in airfoil section Clmax) due to the unsteady variations in angle of attack.
While there can be an increase in the maximum achievable lift, when the rotor blade does stall (i.e., lift stall), the rotor blade can experience a relatively large nose-down pitching moment. The relatively large nose-down pitching moment (i.e., moment stall) which usually precedes the lift stall can cause large vibratory loads in rotor blade controls and the rotor hub. Because of these vibratory loads, the speed, weight, altitude and/or other performance parameters of the rotorcraft may need to be limited so that these high vibratory loads can be avoided. Moreover, flight time in such conditions can reduce the life of the rotor hub and the rotor blade controls and can increase maintenance costs.
Typically, the solidity of the rotor blade can be increased to delay the onset of boundary layer separation, i.e., the stall condition. Increasing rotor solidity can include increasing a chord of the rotor blade or increasing the number of blades. For certain overall weight and/or operating speeds, the increase in the solidity of the rotor blade can reduce a value of a local section lift coefficient (i.e., decrease Cl) at certain local rotor sections below the maximum value of achievable lift (i.e., Clmax). By doing so, the onset of the stall condition can be delayed. While the stall condition can be delayed, the rotor blade can, nevertheless, stall. Moreover, increasing the solidity of the rotor blade can increase the magnitude of the pitching moment of the rotor blade by a square of the chord length (i.e., (pitching moment)˜(chord length)2).
To address the increased magnitude of the pitching moment, the rotor blade airfoils can be implemented with trailing edge tabs and/or a relatively moderate camber. The trailing edge tabs can be set at a negative angle, i.e., upward from the trailing-edge. Alternatively, the rotor blade airfoils can be designed to have negative camber (i.e., reverse camber) in a region of the trailing edge. The various combinations of changes to solidity and camber and the addition of trailing edge tabs can delay the onset of stall and can reduce the magnitude of the pitching moments due to the stall condition.
The various combinations can, however, add to the complexity and weight of the rotor blades especially increasing the number of rotor blades. Increasing the solidity of the rotor blades and/or increasing the number of the rotor blades can require more engine power to overcome increased profile drag produced by the rotor blades, as profile drag can be proportional to the blade area. Increased rotor blade solidity and/or camber and/or solidity can increase the weight of the rotor blades, the rotor hub, the rotor blade controls and associated structures of the rotorcraft. While the above rotor blade configurations remain useful for their intended purposes, there remains room in the art for improvement.